Dark matter binds the galaxy together. According to detailed computer simulations, it also binds galaxies to each other, creating a vast structure often called the "cosmic web" due to its appearance. In these models, the filaments that connect this web are traced by a combination of dark matter and atoms in the form of very low-density gas. While astronomers can identify the nodes of this web in the form of dark matter halos surrounding galaxies, the connecting threads have proven a little more challenging to spot.

Now, a group of astronomers identified one such filament close to a very distant galaxy. Sebastiano Cantapulo and colleagues observed the light emitted by the filament's gas as it glowed under bombardment from a quasar, a powerful jet of particles propelled from a massive black hole. However, the researchers also found at least ten times more gas than expected from cosmological simulations, which suggests that there may be more gas between galaxies than models predict.

The cosmic web as predicted by large-scale computer simulations. The brighter colors in this image indicate larger densities of dark matter, which form nodes and long filaments. The inset shows the region near a galaxy, similar to that of observed in the present study.

Anatoly Klypin and Joel Primack, S. Cantalupo

Early in its history, the Universe had no stars or galaxies, and the density of all matter was remarkably uniform. However, tiny fluctuations in this density—as observed in the cosmic microwave background—led to small regions where the amount of dark matter was slightly higher than elsewhere. Those slight overdensities in turn attracted more matter, producing a slow cascade: some places collected a lot of dark matter and gas, while others were largely emptied out.

According to sophisticated supercomputer simulations, the result was the cosmic web: nodes of dark matter linked by thinner filaments, with vast voids between. (Computer models are necessary because the calculations are far too involved to perform by hand from first principles.) Galaxies and clusters of galaxies formed in the denser regions from gas attracted by the gravitational pull of dark matter. The resulting web is termed the large-scale structure of the Universe, and much of observational cosmology involves the process of mapping this cosmic web in three dimensions. (We see the sky as a two-dimensional surface and must use sophisticated techniques to infer the third dimension: distance from Earth.)

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While the cosmic microwave background provides a picture of the early Universe and galaxy surveys plot out the basic structure of the cosmic web, the filaments connecting everything together are fundamentally difficult to observe. That's because they contain a lot of dark matter, relatively little gas, and few or no stars. Astronomers discovered some filaments by measuring the light absorbed by the gas; others have used gravitational lensing from the dark matter.

The new study used a different trick. Most large galaxies harbor a huge black hole, some of which are (or were) very active, devouring gas and spewing out powerful jets of matter. These jets are easy to spot at great distances, as they emit a lot of high-energy light; we've called these active galaxies "quasars."

That light can also interact with atoms in cosmic filaments, causing them to glow through a process similar to the one used in fluorescent lights. In the case of intergalactic gas, though, the atoms are hydrogen (rather than mercury vapor), and the emitted photons are a specific wavelength of ultraviolet known as Lyman-alpha, or Ly-α. The authors of the new study used a special filter on the Very Large Telescope (VLT), one that was designed for the purpose of spotting Ly-α fluorescence in distant filaments of gas. This method was also used to find a "dark galaxy" that contained very few stars but significant amounts of hydrogen.

The astronomers found a quasar approximately 11 billion light-years away that was powering fluorescence in a clump of gas. This gas is close to the quasar's host galaxy, but it extends far beyond its measured boundaries and contains very few stars. Those facts indicate strongly that it is part of an extragalactic filament, as predicted by the theory of large-scale structure.

However, the amount of Ly-α emission indicated a lot more gas than predicted by computer models—the researchers estimated at least ten times the expected density of gas. That could mean several things: perhaps there are more big clouds of hydrogen in filaments than large-scale structure theory predicts, or perhaps this one particular filament may contain more gas than others for unknown reasons. As usual, the solution will lie in more research—and hopefully the identification of more fluorescing filaments.